The Origin of Near Earth Asteroids

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1 The Origin of Near Earth Asteroids Judit Györgyey Ries Priors, Quaternions and Residuals, Oh My! September 24, 2004 Austin, Texas

2 Outline Why are we interested in Near Earth Asteroids? How does an asteroid become an NEA? The structure of the Asteroid Belt Collisions Mean motion and secular resonances Non-gravitational effects Transport from the Main Belt McDonald Observatory and NEA research Time permitting

3 Asteroids and the Asteroid Belt

4 Structure of the Asteroid Belt Kirkwood (1867) Orbital elements reveal structure at mean motions, where i n A j n J n = (GM/a 3 ) 1/2 i and j are small integers No satisfactory explanation till the mid 1980es and even then

5 Near Earth Objects: NEOs NEOs: Asteroids and comets with q < 1.3 AU NECs: q < 1.3 AU, P < 200 years q = perihelion distance Q = aphelion distance a = semi-major axis NEA groups: Aten: a <1.0 AU, Q > AU (Earth crossers from inside) Apollo: a > 1.0 AU, q < AU (Earth crossers from outside) Amor: a > 1.0 AU, < q < 1.3 AU (Exterior to Earth's orbit but interior to Mars )

6 More Definitions PHAs - Potentially Hazardous Asteroids Minimum orbit intersection distance with the Earth 0.05 AU Chance to get closer to Earth than 20 lunar distances Absolute magnitude is H= 22.0 or brighter. H is defined as the mean brightness at zero phase angle 1 AU from the Earth and the Sun Estimated size D log(d) = log(p) - 0.2H 0.05 p H D m m m 1 magnitude uncertainty in H introduces a factor of 2 error in D, corresponding to a factor of 8 in impact energy

7 Terrestrial Impact Structures Geological evidence for old collisions: Impact structures Iridium abundance

8 Observed Events 1908 Tunguska Valley 2000 km 2 flattened, seismic vibrations recorded as far away as 600 miles At 300 miles loud bangs heard, a fiery cloud on the horizon At 110 miles brilliant fireball seen with 500 mile tail, thunderous noises reported At 40 miles people were thrown to the ground, knocked unconscious; windows broken Magnetic storm after the event, unusually bright night all over the world 1935 British Guyana? Native legends 1947 Sikhote-Alin 1992 Peekskill, New York 2003 Chicago, Illinois

9 But the real reason to monitor NEAs is because.no one is completely safe

10 Number of Known NEAs Largest NEA is 25 km in diameter, majority is less than 1 km in size At present, we do not know of any NEA which is actually destined to hit the Earth Of the 55 objects having the highest collision probability, the three largest are ~ 700m One object requires careful monitoring two potential impacts in 2101 size ~230m

11 So, is IT coming, and when? None that we know of at the moment The last big one (~10-15 km) came 65 million years ago The population of hazardous objects is unknown Estimated % of large asteroids is still undiscovered) Amount of damage by a given impactor is uncertain

12 How do Asteroids Become NEAs? Mean motion resonances with Jupiter Eccentricity of the asteroid grows large, leading to collisions with neighbors and ejecting fragments Eccentricity of the asteroid grows large, no collisions with neighbors, becomes terrestrial planet crosser (most end up in the Sun) Ejection from the inner solar system Secular resonance with major planets In the case of secular resonance, what matters are not the orbital periods, but rather the periods of time (on the order of tens of thousands of years) over which the orbits change their mutual orientation The ν 6 resonance affects orbits whose direction of perihelion precesses around the Sun at the same rate as Saturn Combined with the 4:1 mean motion resonance, it provides the inner and high-inclination boundary to the observed distribution of asteroids Steady provider of chaotic Earth crossing orbits

13 Example of a mean motion resonance Energy exchange tends to be in same direction at conjunction is slowly varying - terms depending on this angle no longer time-average to zero THEY ARE CAPABLE OF EXCHANGING ENERGY

14 How do Asteroids Become NEAs? Median lifetime for a resonant asteroid before it becomes Earth crosser 3:1 resonance, few million years cannot be the only source, we know meteorite ages up to 20 million years 2:1, hard to remove bodies, most of them ejected on hyperbolic orbits 5:2, lifetime a few 100,000 years, but most of them ejected ν 6, 2 million (6 million as NEA) shorter than the age of the Solar System Needs replenishing, cratering rates indicate steady NEO flux over the last 3 billion years We can look at overlapping resonances, diffusion, resonance with terrestrial planets, close encounters with Mars Helps but still does not explain steady flux or old meteorites What about non-gravitational forces?

15 Solar Radiation Effects Radiation pressure - no secular effect Poynting-Robertson drag - more applicable to dust Yarkovsky effect - delayed re-radiation of absorbed Solar radiation affects sizes few meters to 20 km Diurnal and seasonal Yarkovsky effect Additional perturbation due to surface inhomogeneities and irregular shapes (YORP effect)

16 Yarkovsky Effect (Delayed reradiation of heat)

17 LAGEOS - EARTH THERMAL HEATING Delayed reradiation of heat absorbed from the Earth results in a non-zero net transverse acceleration that decreases the semimajor axis Effect is maximum when the spin axis is in the orbital plane (leads to periodic variations as orbit plane precesses) 10 meters in 28 years from Rubincam, 1987

18 Yarkovsky-type Accelerations for LAGEOS 4 2 picometers/sec/sec) Years past 1 Jan 1976 Drag-like forces observed on LAGEOS soon after launch that was several times larger than expected from drag, reducing semi-major axis by ~37 cm/yr. Eventually, it became clear that Yarkovsky-type forces were the cause.

19 Origin of NEOs - Summary Long term numerical simulation (based on the debiased NEO population, distribution of main belt rotation rates, assuming thermal properties) 23% form 3:1 resonance 25% from Mars crossers 37% from ν 6 secular resonance 8% Diffusive resonances in the outer belt 6% trans - Neptunian While these regions deplete on the order of 10 million years the Yarkovsky effect can move collisional fragments to these region to provide a steady source.

20 NEO McDonald Observatory Why bother with follow-up observation? Orbits for confirmation objects and provisional designations are based on a limited number of observations: Short arc Limited time coverage Only gravitational effects inc. Orbital prediction are limited, some NEAs are lost due to insufficient follow-up 2004/ out of the 151 new objects posted were not confirmed, 3 were not real, and 51 were not interesting. 0.7m telescope with prime focus camera (22 nd magnitude in R in 15 minute exposure)

21 NEO McDonald Observatory We take a set of three CCD images with the R filter, on each plate: Match stellar images with positions calculated from coordinates given in USNO-A2.0 Determine plate solution x = a 1 x + a 2 y + a 3 + a 4 xy + a 5 (x 2 +y 2 ) + a 6 x(x 2 + y 2 ) h = b 1 y + b 2 x + b 3 + b 4 xy + b 5 (x 2 +y 2 ) + b 6 y(x 2 + y 2 ) Measure and calculate target position Accuracy from residuals provided by MPC is about arcsec Using USNO-A2.0 to provide stellar magnitudes in R, we can achieve an accuracy of about mag

22 Rotation period determination Measuring asteroid brightness in R Brightness changes Distance of asteroid from Sun and Earth changes Amount of surface reflecting light changes Surface reflectivity changes We can determine Rotation rate Shape Pole orientation Surface reflectivity

23 Yarkovsky-O keefe keefe-radzievskii-paddack (YORP) Effect Simple model of an asymmetric asteroid: Rotating BB sphere with wedges at the equator emit radiation in opposite directions providing a torque Asteroids spins up or really slows downs We can measure asteroid rotation periods

24 Lightcurves McDonald Observatory P= 1.16 h Peak to peak variation 0.15 mag P= 2.49 h Peak to peak variation 0.95 mag P= 2.22* h Peak to peak variation 0.2 mag Arecibo radar target, binary asteroid